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Hemophilia Inhibitors 101

Thanks to the widespread use of clotting factor, people with hemophilia today are able to live much more active and full lives compared with just a few decades ago. However, some people with hemophilia and von Willebrand disease (VWD) who infuse factor develop a complication known as an inhibitor. What are inhibitors and who is at risk of developing them? See below to learn the answers to these and other key questions.

What are inhibitors?

Inhibitors are antibodies created by the immune system. These antibodies attack proteins in clotting factor, stopping the factor from doing its job of preventing and treating bleeds.

Who gets inhibitors?

Anyone with hemophilia A or B, or VWD type 3, who infuses clotting factor concentrates may develop an inhibitor. Most often inhibitors develop during childhood following the first several infusions of factor, but they can occur at any age.

How common are inhibitors?

Inhibitors are more common in people with hemophilia A. Twenty percent to 30% of people with severe hemophilia A will develop an inhibitor to factor VIII, while 5% to 8% of people with mild or moderate hemophilia A will develop them. Among people with hemophilia B, roughly 4% will develop inhibitors to factor IX. And in people with VWD type 3, 7.5% to 9.5% will develop inhibitors to infused von Willebrand factor.

What are the symptoms of inhibitors?

An inhibitor may be suspected if a bleed isn’t controlled with the usual dose of factor or if a person has frequent bleeds despite being on a regular treatment schedule (what’s known as breakthrough bleeding).

Who has an increased risk of developing an inhibitor?

The following groups are more likely to develop inhibitors: people with severe hemophilia; people of African-American and Hispanic descent; people who have a family history of inhibitors; people with certain genetic mutations of hemophilia.

How is an inhibitor diagnosed?

A simple blood test measures if an inhibitor is present and the amount of inhibitor (called a titer) in the blood. Since 2015, the Centers for Disease Control and Prevention (CDC) and the National Hemophilia Foundation’s Medical and Scientific Advisory Council (MASAC) have recommended that all people with hemophilia, regardless of age, be tested at least once a year for inhibitors. Annual inhibitor screening is available free of charge at federally funded hemophilia treatment centers (HTCs) through the Community Counts program.

How are inhibitors treated?

Treating inhibitors requires close monitoring by the medical team at HTCs. The first goal is to manage bleeds despite the presence of the inhibitor. If a person has low inhibitor levels, using a greater amount of factor and additional doses may be enough to control bleeds. In people with high levels of an inhibitor, medications known as bypassing agents are used to treat bleeding episodes. Bypassing agents are special clotting factors that “bypass” the body’s need for factor VIII or factor IX to form clots.

The second, longer-term goal of inhibitor treatment is to eradicate the inhibitors from the bloodstream. This is accomplished through immune tolerance induction (ITI), in which the immune system is trained to accept FVIII or FIX clotting factor. ITI requires steadily increasing the amount of factor a person receives and the frequency of doses. Depending on the individual, ITI may take weeks, months or even years to work.

Also, new treatments are in development. One, Hemlibra, was approved in November 2017 by the US Food and Drug Administration (FDA) for use in people with hemophilia A who have FVIII inhibitors. Hemlibra is injected subcutaneously (under the skin) once a week.

Learn more about inhibitors

NHF Inhibitor Education Summits

NHF holds three-day, comprehensive educational programs for people with hemophilia A or B with inhibitors and their caregivers. Find out more about the summits.